1. Field of the Invention
The present invention relates to a high-frequency transmitting/receiving apparatus employing a nonradiative dielectric waveguide (an NRD guide) for use in a millimeter-wave radar module, a millimeter-wave wireless radio communication apparatus, or the like, and more particularly to a high-frequency transmitting/receiving apparatus having a switching device for avoiding that a pulse-modulated millimeter-wave signal for transmission is outputted to a reception system due to inner reflection or other causes, wherein the influence of switching noise occurring in the switching device can be reduced. The invention also relates to a radar system having the high-frequency transmitting/receiving apparatus, a vehicle equipped with the radar system, and a small boat equipped with the radar system.
2. Description of the Related Art
Conventionally, some types of high-frequency transmitting/receiving apparatuses have been proposed that are expected to find applications in a millimeter-wave radar module, a millimeter-wave wireless radio communication apparatus, or the like. For example, Japanese Unexamined Patent Publication JP-A 2000-258525 discloses a high-frequency transmitting/receiving apparatus adopting a pulse modulation method.
However, in the pulse modulation method, part of a pulse-modulated millimeter-wave signal for transmission is outputted to a reception system, as an unwanted signal, due to reflection or other phenomena occurring within the high-frequency transmitting/receiving apparatus. This gives rise to a problem of the reception capability being degraded.
In this respect, the inventors of the present application have already come up with a solution to the aforementioned problem (refer to Japanese Unexamined Patent Publication JP-A 2002-328161). FIGS. 16 and 17 are plan views showing embodiments of the construction disclosed therein. Note that the NRD guide employed in the embodiments has such a basic structure as shown in a partly cutaway perspective view of FIG. 18. That is, a dielectric line 43 is interposed between a pair of parallel plate conductors 41 and 42.
FIG. 16 shows one embodiment of the high-frequency transmitting/receiving apparatus in which a transmitting antenna and a receiving antenna are integrated with each other. The high-frequency transmitting/receiving apparatus comprises a pair of parallel plate conductors 51; a first dielectric line 53; a millimeter-wave signal oscillator 52; a pulse modulator (not shown); a second dielectric line 58; a circulator 54; a third dielectric line 55; a fourth dielectric line 57; and a mixer 59. The pair of parallel plate conductors 51 are disposed at an interval equal to or less than one half of the wavelength of a millimeter-wave signal. The first dielectric line 53 is provided between the parallel plate conductors 51. The millimeter-wave signal oscillator 52 is provided between the parallel plate conductors 51 and attached to the first dielectric line 53. The millimeter-wave signal oscillator 52 converts a high-frequency signal outputted from a high-frequency diode to a frequency-modulated millimeter-wave signal, and allows the millimeter-wave signal to propagate through the first dielectric line 53. The pulse modulator is provided between the parallel plate conductors 51 and disposed at a midway position of the first dielectric line 53. The pulse modulator puts the millimeter-wave signal out, as a pulsed millimeter-wave signal for transmission, from the first dielectric line 53. The second dielectric line 58 is provided between the parallel plate conductors 51, disposed in proximity to the first dielectric line 53 in such a way that one end thereof is electromagnetically coupled with the first dielectric line 53, or coupled at one end thereof with the first dielectric line 53, and allows a part of the millimeter-wave signal to propagate toward the mixer 59. The circulator 54 is provided between the parallel plate conductors 51 and has a first connection portion 54a, a second connection portion 54b, and a third connection portion 54c. These connection portions, which are arranged at predetermined intervals about the periphery of a ferrite plate disposed in parallel with the parallel plate conductors 51, respectively act as millimeter-wave signal input and output ends. In the circulator 54, a millimeter-wave signal inputted from one of the connection portions is outputted from the other connection portion adjoining in a clockwise or counterclockwise direction within the plane of the ferrite plate. The first connection portion 54a is connected to the millimeter-wave signal output end of the first dielectric line 53. The third dielectric line 55 is provided between the parallel plate conductors 51 and connected to the second connection portion 54b of the circulator 54, allows a millimeter-wave signal to propagate therethrough and has a transmitting/receiving antenna 56 at the front end thereof. The fourth dielectric line 57 is provided between the parallel plate conductors 51 and connected to the third connection portion 54c of the circulator 54. The fourth dielectric line 57 allows a reception wave, which has been received by a transmitting/receiving antenna 56, propagated through the third dielectric line 55, passed through the second connection portion 54b, and outputted from the third connection portion 54c, to propagate toward the mixer 59. The mixer 59 is provided between the parallel plate conductors 51 and constructed by proximately placing or coupling a mid-portion of the second dielectric line 58 and a mid-portion of the fourth dielectric line 57 so as to achieve electromagnetic coupling therebetween. The mixer 59 mixes a part of a millimeter-wave signal and a reception wave to generate an intermediate-frequency signal. Besides, in this example, a switching control section (not shown) is disposed at the output end of the mixer 59. The switching control section turns the output end into an opened state at the time when a pulse-modulated millimeter-wave signal for transmission is outputted from the pulse modulator. Thereby, it is possible to prevent an unwanted signal from being outputted to a reception system located downstream of the mixer 59 substantially concurrently with inputting of a pulsed signal for starting a pulsing operation in the pulse modulator to the pulse modulator.
FIG. 17 shows another embodiment of the high-frequency transmitting/receiving apparatus in which a transmitting antenna and a receiving antenna are provided separately. The high-frequency transmitting/receiving apparatus comprises a pair of parallel plate conductors 61; a first dielectric line 63; a millimeter-wave signal oscillator 62; a pulse modulator (not shown); a second dielectric line 68; a circulator 64; a third dielectric line 65; a fourth dielectric line 69; a fifth dielectric line 67; and a mixer 71. The pair of parallel plate conductors 61 are disposed at an interval equal to or less than one half of the wavelength of a millimeter-wave signal. The first dielectric line 63 is provided between the parallel plate conductors 61. The millimeter-wave signal oscillator 62 is provided between the parallel plate conductors 61 and attached to the first dielectric line 63. The millimeter-wave signal oscillator 62 converts a high-frequency signal outputted from a high-frequency diode into a frequency-modulated millimeter-wave signal, and allows the millimeter-wave signal to propagate through the first dielectric line 63. The pulse modulator is provided between the parallel plate conductors 61 and disposed at a midway position of the first dielectric line 63. The pulse modulator puts the millimeter-wave signal out, as a pulsed millimeter-wave signal for transmission, from the first dielectric line 63. The second dielectric line 68 is provided between the parallel plate conductors 61, disposed in proximity to the first dielectric line 63 in such a way that one end thereof is electromagnetically coupled with the first dielectric line 63, or coupled at one end thereof with the first dielectric line 63, and allows a part of the millimeter-wave signal to propagate toward the mixer 71. The circulator 64 is provided between the parallel plate conductors 61 and has a first connection portion 64a, a second connection portion 64b, and a third connection portion 64c. These connection portions, which are arranged at predetermined intervals about the periphery of a ferrite plate disposed in parallel with the parallel plate conductors 61, respectively act as millimeter-wave signal input and output ends. In the circulator 64, a millimeter-wave signal inputted from one of the connection portions is outputted from the other connection portion adjoining in a clockwise or counterclockwise direction within the plane of the ferrite plate. The first connection portion 64a is connected to the millimeter-wave signal output end of the first dielectric line 63. The third dielectric line 65 is provided between the parallel plate conductors 61, connected to the second connection portion 64b of the circulator 64, allows a millimeter-wave signal to propagate therethrough and has a transmitting antenna 66 at the front end thereof. The fourth dielectric line 69 is provided between the parallel plate conductors 61 and has a receiving antenna 70 at the front end thereof, and has the mixer 71 at the other end thereof. The fifth dielectric line 67 is provided between the parallel plate conductors 61 and connected to the third connection portion 64c of the circulator 64, and has a reflectionless terminator 67a disposed at the front end thereof. The reflectionless terminator 67a acts to attenuate a millimeter-wave signal which has been intrusively received at the transmitting antenna 66. The mixer 71 is provided between the parallel plate conductors 61 and constructed by proximately placing or coupling a mid-portion of the second dielectric line 68 and a mid-portion of the fourth dielectric line 69 so as to achieve electromagnetic coupling therebetween. The mixer 71 mixes a part of a millimeter-wave signal and a reception wave to generate an intermediate-frequency signal. Besides, in this example, a switching control section (not shown) is disposed at the output end of the mixer 71. The switching control section turns the output end into an opened state at the time when a pulse-modulated millimeter-wave signal for transmission is outputted from the pulse modulator. Thereby, it is possible to prevent an unwanted signal, which directly intruded from the transmitting antenna 66 into the receiving antenna 70, from being outputted to a reception system located downstream of the mixer 71 substantially concurrently with inputting of a pulsed signal for starting a pulsing operation in the pulse modulator to the pulse modulator.
Next, FIG. 19 is a block circuit diagram showing the structure of each constituent component of the high-frequency transmitting/receiving apparatus shown in FIG. 16, which is implemented as a millimeter-wave radar.
In FIG. 19, reference numeral 111 represents a VCO equipped with a Gunn diode and a varactor diode. The VCO 111 is activated by inputting a signal to its IN-2 terminal for doing input of a modulation signal. A signal outputted from the VCO 111 and a pulsed signal inputted to an IN-1 terminal are inputted to a pulse modulator 112, thereby achieving pulse modulation. The pulse modulator 112, which is disposed at a midway position of the first dielectric line 53 in FIG. 16, is built as a switch (RF switch) having such a structure as perspectively illustrated in FIG. 20.
The pulse modulator shown in FIG. 20 is constructed as follows. A choke-type bias supply line 90 is formed on one main surface of a wiring board 88. In the midway thereof are formed connection electrodes 81. A beam-lead type PIN diode or a Schottky-barrier diode 80 is mounted by soldering midway between the connection electrodes 81. The PIN diode or the Schottky-barrier diode 80 is placed midway between the end faces of the first dielectric line 53 in such a way that the bias voltage applying direction coincides with the lateral direction. Such a switch is used as the pulse modulator 112.
Reference numeral 113 represents a circulator for transmitting a millimeter-wave signal toward an antenna 114 during transmission, while transmitting a reception wave toward a mixer 115 during reception. Reference numeral 114 represents a millimeter-wave signal transmitting/receiving antenna. The antenna 114 is connected to the circulator 113 via a metal waveguide or a dielectric waveguide composed of a dielectric-filled metal waveguide. For example, the antenna 114 may be built as a horn antenna. Reference numeral 115 represents a mixer for mixing a millimeter-wave signal outputted from the VCO 111 and a reception signal received at the antenna 114 to generate an intermediate-frequency signal required to detect the distance to a target object.
Reference numeral 116 represents a switch for interrupting and passing alternately the intermediate-frequency signal outputted from the mixer 115. Reference numeral 119 represents a control section for controlling switching timing of the switch 116 (ON-OFF timing). The switch 116 and the control section 119 constitute a switching control section.
The control section 119 controls the ON-OFF timing as follows. When a pulsed signal is inputted to the IN-1 terminal in synchronization with the pulse modulator 112, a millimeter-wave signal for transmission that has been pulse-modulated by the pulse modulator 112 may be reflected from the connection between the NRD guide and the dielectric waveguide, or may leak from the circulator 113, with the result that the millimeter-wave signal is outputted as an unwanted signal through the mixer 115. Before the unwanted signal is directed to an amplifier 118, the control section 119 drives the switch 116 to interrupt the unwanted signal.
Note that reference numeral 117 represents a capacitor for achieving alternating-current coupling between the switch 116 and the amplifier 118.
According to the constructions such as shown hereinabove, it is possible to avoid that a pulse-modulated millimeter-wave signal for transmission enters the mixer 115 and resultantly leaks into a downstream-side reception system. As a result, the millimeter-wave radar system will succeed in providing enhanced detection accuracy.
On the other hand, another conventional example of a high-frequency transmitting/receiving apparatus adopting a pulse modulation method is disclosed in Japanese Unexamined Patent Publication JP-A 2003-198421. The high-frequency transmitting/receiving apparatus is provided with reception inhibitory means which is analogous to the switching control section as described above. In this construction, millimeter-wave signals for transmission are intermittently transmitted by an RF switch or the like means. During a pause in transmission of the millimeter-wave signals for transmission, intermediate-frequency signals are interrupted to suspend reception.
However, in order to achieve further enhancement of the performance of the high-frequency transmitting/receiving apparatus disclosed in JP-A 2002-328161, the inventors of the present application have conducted diligent, extensive research and study, and resultantly found the following problems to be addressed.
At first, in the switch 116, the timing of switching, at least the closing (ON) timing needs to be controlled with high accuracy.
In general, the pulse modulator 112 using a high-frequency diode possesses characteristics inherent in high-frequency diodes such as a zero bias capacitance. Therefore, even if a pulse signal ideal for driving is inputted, a distortion such as ringing noise may appear in modulation current to a greater or lesser degree. Furthermore, the pulse signal for driving itself may suffer from a similar distortion in varying degrees. In view of this, a certain period of time is required to stabilize the output intensity of the millimeter-wave signal outputted from the pulse modulator 112 in 1 signal period. As a result, if the switch 116 is turned into a closed (ON) state after the signal at the IN-1 terminal is made to be an opened (OFF)-state signal so that the millimeter-wave signal may enter a state of an output-OFF state, depending upon the ON-OFF timing, there is a possibility that variation in the output intensity of the millimeter-wave signal still remains at that time, with the result that an unwanted signal (noise) may be outputted to the mixer 115 and mixed with proper signals to be detected. This leads to degradation in the radar detection performance.
Next, in the millimeter radar shown in FIG. 19, a timing signal indicating switching timing of the switch 116 is generated under the control of the control section 119. At this time, if the switching timing signal is generated by using only the signal at the IN-1 terminal, there is a possibility that the ON-OFF timing cannot be controlled with high accuracy, or that the circuit for generating the timing signal is undesirably complicated.
In the millimeter radar shown in FIG. 19, switching of the switch 116 with use of the signal at the IN-1 terminal is performed as follows. At first, an output of the millimeter-wave signal from the pulse modulator 112 is controlled on the basis of the signal at the IN-1 terminal (the millimeter-wave signal is outputted while the IN-1 terminal is kept in a closed (ON) state). Thereby, the switch 116 can be turned into a closed (ON) state by exploiting the timing with which the signal at the IN-1 terminal changes from a closed (ON) state to an opened (OFF) state. Next, timing to turn the switch 116 into an opened (OFF) state will be described. In the case of using the signal at the IN-1 terminal, the switch 116 needs to be turned into an opened (OFF) state before the signal at the IN-1 terminal, now changed from a closed (ON) state to an opened (OFF) state, changes from an opened (OFF) state to a closed (ON) state once again. Therefore, as a timing signal indicating timing to turn the switch 116 into an opened (OFF) state, a signal obtained by delaying the signal at the IN-1 terminal by a certain time interval is required. Such a signal cannot be generated without using a time-delay circuit or the like. For example, a CR delay circuit is desirable from the standpoint of convenience.
However, the millimeter radar shown in FIG. 19 poses the following problems. In a pulse signal which is applied to the IN-1 terminal and is then inputted to the pulse modulator 112, the pulse cycle is longer relative to the pulse width. This requires much delay time in the CR delay circuit, with the result that the timing may be greatly varied even if variation in the circuit constant is slight. Furthermore, with the addition of the CR delay circuit, the entire circuit configuration is undesirably complicated.
Moreover, in order to achieve further enhancement of the performance of the high-frequency transmitting/receiving apparatus disclosed in JP-A 2002-328161, the inventors of the present application have conducted diligent, extensive research and study, and resultantly found the following problems to be addressed.
One of the problems is that switching noise is produced in accompaniment with switching of the switch 116. The influence of the switching noise exerted on the other circuit systems has to be minimized.
In general, the switch 116 is required to operate at high speed in response to control signals. In view of this, an analog switch such as a CMOS is employed. However, because of its property, the switch 116 incurs switching noise, though slight, during switching operations. Inconveniently, the switching noise is amplified by the amplifier 118 located on the downstream side, and finally finds its way into the other circuit systems nearby as an unwanted signal so as to have adverse effect thereon.
Next, it is preferable that, when applied to a millimeter-wave radar, the high-frequency transmitting/receiving apparatus is provided with a self-monitoring function for detecting and notifying abnormality occurring in the circuit system. By doing so, for example, in the case of applying the high-frequency transmitting/receiving apparatus to a vehicle-mounted collision avoidance radar, it is possible to notify in advance an operator about a malfunction occurring in the circuit system of the high-frequency transmitting/receiving apparatus. This makes it possible to avoid the danger of causing an accident due to unawareness of a malfunction occurring in the vehicle-mounted collision avoidance radar. Note that the self-monitoring function should preferably be constituted without complicating the circuits, without having any adverse effect on the basic performance of the high-frequency transmitting/receiving apparatus, and without any difficulty.
Another problem is that, while the switch 116 is kept in an opened (OFF) state, a load impedance appears to be infinite in terms of an output from the mixer 115 (corresponding to an extremely-low-capacity open end), and thus the high-frequency component of the output from the mixer 115 is substantially totally reflected toward the mixer 115.
As a result, the mixer 115 is brought into malfunction, and besides, a part of a multiple-reflected central frequency signal may find its way into the circuits located downstream of the switch 116, which ends in failure of proper outputting of intermediate-frequency signals.
Moreover, in the technique disclosed in JP-A 2003-198421, timing to suspend a reception operation needs to be controlled with high accuracy in the reception inhibitory means. If the reception inhibition timing is not controlled with high accuracy, the following problem arises. Since a certain period of time is required for a millimeter-wave signal for transmission to be returned from a to-be-detected target object through reflection, it follows that a millimeter-wave signal to be received may be inputted, even during a pause in transmission of the millimeter-wave signal for transmission. If the reception operation is suspended at this time, there is a possibility that important information to be received is missed.